The H+-translocating ATPase (ATP synthase) is a large, complex enzyme that carries out the conversion of ADP and inorganic phosphate to ATP coupled to the translocation of protons. The enzyme utilizes the electrochemical gradients of protons maintained by the oxidation of reduced substrates to synthesize ATP at the high concentrations needed to carry out all energy requiring cellular processes. The ATP synthase has been the subject of extensive biochemical experimentation, but progress in the field has been hampered by the lack of detailed three- dimensional structural information. In this project it is proposed to use single crystal x-ray diffraction methods to obtain a molecular model of the F1-sector of the ATP synthase of rat liver mitochondria. The crystals to be used in this project are the only available ATP synthase material suitable for x-ray crystallographic studies. Their diffraction pattern shows reflections with spacings up to 3.5 Angstroms and data to this resolution can be obtained using synchrotron radiation. An electron density map calculated with 3.5 Angstroms resolution data will be used to trace the polypeptide chains and to provide a molecular model of the F1-sector of the ATP synthase. Crystals will be prepared with different amounts and types of nucleotides present and the location of their binding sites will be determined using x-ray diffraction data and difference Fourier methods. The molecular model of the F1-ATPase and the location of the nucleotide binding sites will be of great value for organizing the existing biochemical information and for directing the future biochemical experimentation towards a complete understanding of the molecular mechanism of ATP synthesis.